Booms of this type are known from practice, for instance from mobile cranes. A load is hung on the front end of an extended inner structure so that the inner structure with its lower flange exerts a load on the front end section of the lower flange of the outer structure. At the same time, the rear end of the inner structure exerts a load on the upper flange of the outer structure with its upper flange. Within the scope of this invention, "front" designates the direction towards the load-receiving free end of the boom, and "rear" designates the direction towards the end opposite to the free boom end.
On account of the great forces to be transmitted, special attention has to be paid to the design of the front and rear bearings connected to the inner and outer structures. Apart from a reliable support, these bearings are also designed to counteract undesired deformations of the profile cross-sections of the inner and outer structures. Normally, full support of slide elements of the front and rear bearings is therefore sought to be achieved. The slide elements are each fixed on one side to a laterally bordered plane bearing block on one of the inner or outer structures and the slide elements are each supported on the adjoining outer or inner structure at the opposite side of the respective slide element. Reinforcing support means in the form of collars are required for receiving the forces acting on the slide elements and the bearing blocks. As a rule, the collars on the inner structures for the rear bearings are made continuous over the cross-sectional shape of the inner structure and jointly form an enclosure for the slide elements, whilst the collars of the front bearings are normally mounted externally on the outer structure and consist of solid material with a dimension of 150 to 300 mm in the longitudinal direction of the boom. The collars thereby reduce the usable extension length of the structure in question. As shown in FIG. 6 (prior art), the inner structures cannot be fully telescoped into one another, as they are hindered from doing so by the inner collars. The same happens analogously in the region of the front bearing with the outer collars.
DE-OS 1531174 suggests a roller bearing for telescopic booms with a polygonal cross-section. To receive the edge stresses occurring upon load, each edge of the lower flange has assigned thereto an outer roll, and each edge of the upper flange an inner roll. The outer and inner rolls present obstacles to one another in a disadvantageous manner, whenever the structures are slid into one another, so that the usable extension length of the structures is reduced at least by the sum of the diameter of the rolls arranged side by side. Moreover, the inner rolls occupy a lot of space in the hollow profile cross-section of the inner structure, so that there is only little space for the telescopic cylinder unit arranged therein for extending the boom.